Complex integrated behavior is primarily the result of processes in the central nervous system of an organism. However, the peripheral sensory system plays a decisive role in that it provides a basis on which the CNS can operate. Furthermore, the peripheral sensory organs may determine which behavior will be expressed. One behavior that appears to be regulated, at least in part, by the activity of a peripheral receptor system in female mosquitoes is host-seeking behavior--a behavior directly related to the transmission of diseases to man and his animals. In the current research, we have been investigating the humorally induced desensitization of the receptors sensitive to the host attractant, lactic acid (LA), and the subsequent inhibition of host- seeking behavior following a blood meal. We identified the source of the inhibitory factor as the fat body and have preliminary evidence that the ovarian ecdysteroids are responsible for triggering the synthesis and/or release of the inhibitor. In addition, we have postulated a new theory for the interaction of the LA-excited and LA-inhibited neurons in the modulation and control of host-seeking behavior in the proposed research. We intend to extend these investigations to the next stage in the post-blood meal inhibition of host-seeking behavior, specifically the maintenance of the inhibition and the recovery of host- seeking activity following oviposition. We also intend to examine the activity and potential interaction of other receptor systems with host-seeking behavior. The ultimate goal in this research is to gain a better understanding of the mechanisms underlying the regulation and control of host-seeking and other behaviors of female mosquitoes. The information developed here should aid in determining how these mechanisms might be employed in a strategy to reduce the potential for disease transmission by these important vector organisms.